This invention relates to measurement of steady-state evoked potentials from the nervous system measured on the body by means which substantially reduces background noise and does not alter the evoked potential.
Steady-state evoked potentials are caused by a rapidly repeated periodic tactile, auditory, optical or other stimulus, and are of great medical value, first to reveal specific brain activities in which different types of information are handled simultaneously in separate channels, second to provide an objective indicator of sensory function where perceptual tests are impractical, as they are with infants and animals, and third to distinguish organic from psychogenic disorders.
A serious difficulty in the use of apparatus of the prior art is to detect the relatively weak evoked potentials in the presence of much stronger background noise due to the general electrical activity of the nervous system. The measurement means of the prior art uses highly selective circuits at the fundamental frequency of the stimulas and at each higher harmonic in order to pass the evoked potential and stop background noise. This is unsatisfactory in that it restricts the response of the apparatus to a number of narrow bands, which may be narrower than the evoked potentials, on the probably erroneous assumption that the evoked potentials are simple sine functions with harmonic frequency ratios. If an evoked potential consists of two or more sine functions not harmonically related this method of measurement is inadequate and misleading. It seems probable that in many cases the stimulus in not a simple sine function, and that the prior art measurement means are largely responding to the characteristics of the stimulus. Even when the stimulas is a sine function the evoked potential may involve amplitude modulation, angle modulation, amplitude limiting and other processes that render the Fourier statement of the evoked potential, as given by measuring instruments of the prior art, incomplete and perhaps inaccurate. In addition, filters narrow enough to block substantially all noise have a very slow response. Apparatus according to this invention substantially removes all noise, from a band including at least one harmonic component of the evoked potential, without materially altering the amplitude or phase of closely associated components, such as sidebands, in the same band as a harmonic component.
We do not know of any relevant prior art.